The invention herein relates to friction materials for brakes suitable for railroad service.
Over the years a number of different types of composition railroad brake shoes have been described. These have normally been composed of a rubber and/or resin matrix heavily reinforced with asbestos fiber and containing lead in the form of elemental lead and/or lead oxides and friction modifiers such as iron chromite, silicon carbide and the like as critical components. Typical compositions have been described in U.S. Pat. Nos. 2,686,140; 2,861,964; 3,152,099; 3,390,113; 3,832,325 3,959,194 and 4,178,278. Many of the materials described in the afore-mentioned patents have enjoyed widespread commercial success as railroad brake shoe compositions under the trademark COBRA. However, environmental and health concerns have led purchasers and therefore manufacturers to seek friction material compositions which contain neither asbestos fiber nor lead metal or its compounds. Recently, a composition containing little or no lead was disclosed. This was found to be quite advantageous in reducing wheel wear, particularly where softer steel wheels are used. See the aforesaid U.S. Pat. No. 3,959,194. Efforts to eliminate asbestos fiber from the brake shoe compositions have been beset with major difficulties, primarily because the asbestos fiber contributed a unique combination of reinforcement and thermal properties to the compositions. During service application brake shoes undergo significant physical stresses and also experience substantial temperature increases, often reaching peak temperatures in excess of 800.degree. F. Previously, fibrous materials other than asbestos such as certain organic fibers, steel fibers and combinations of steel fiber and metal grit have been found suitable to adequately withstand the thermal and physical forces involved in railroad service.
Conventional AAR (Association of American Railroads) service for standard freight cars constitutes the large majority of the uses of friction materials and therefore many of the previous compositions were directed to such end use. Consequently, a basic friction material composition for molded brake shoes which is asbestos-free and suitable for use in AAR service has been sought, which would meet the service requirements and be economical to manufacture.